X-ray meter.



N0.s9s,244. PATENTED JULY 14,1908. G.G.JOHVNSTON.-

X-R'AY METER. Arrmouron FILED JUNE 30.4900.

Selenium can INVENT.

legheny and State of Penns GEORGE G. JOHNSTON, OF PITTSBURG,

PENNSYLVANIA.

To all whom it may concern:

Be it known that I, GEORGE C. JOHNSTON, a resident of. Pittsburg, inthecounty of Al- Ivania, have invented a new and useful mprovement inX-Ray-Meters; and I do hereby declare the following to be a' full,clear, and-exactdeenergy, suc

" -parisonof a chang'e'o Q trati-ng power of the'.'fifays.- 'hav' scrition thereof.

his invention is for apparatus forgmeasuring the uantity orintensity ofradiant as the so-called' Roentgen or -rays.

' The object of the invention is to i rovide apparatus for thispurposein which-the,

quantity or intensity ofxthe radiant energy can be' obtained from adirect readin given by a pointer moving over a graduate scale, thusgivin an instantaneous reading, and without t e necessity ofcalculations or analyses, as has heretofore been necessary. In theaccompanying drawings Figure 1 1s a perspective diagram of my apparatus;Fig. -2is a diagram of the electric circuits. Attempts have been made todevise means whereby an accurate measurement of the qualit and quantityof the invisible radiations nown as X-raysmight be measured.

All prior devices of this kind either re uired calculations to be madefrom the erent readlngs the .measurement de, end ngjupo av ail S lts heo e. th arbitrary color scale-, This' o ers. latitude for error.Estimations;

made by holding in. the path off't various objects, usually the handofthe server, and this practiceis responsiblefor serious physicalinjuries, but as it is thebest way of actuall determining the qualityand quantit of the invisible radiations, it is widely ollowed. In laceof the observers hand, various other ol jects ossessing a certain degreeof opacity to t e X-rays 'have beenused. The readings so obtained areinaccurate, since they are required to be made in the dark, and theobservers e es must be accustomed to the darkness; e'nce their accuracydepends upon the condition of the observer's eyes at the 'timethereading is made, and no two observers read the same degree ofpenetration.

s ecifie of Letters Patent. Application med June so, 1906. Serial No.a24,27a.

- the intensitg cell is exposed. 'tity andintensity of the rays whichhave the power, of rendering fluorescent the salts .sT eeifie'da Theserays are the so-called ositioiis.

in orderto obtain. the, trueg ii g orthe lntensity of the ray wg 'meggits efiect upon certain measure 3 rays.

Patented July 14, 1908.

It is Well known that X-rays have the property of rendering luminous orfluorescent certain salts, such as tungstate of calcium, barium platinocyanid, and others, so that if these salts are observed in darknessunder the action of the X-rays they become brilliantly luminous, and theluminosity bears a constant ratio to the intensity of the rays,

and the distance from the point of emission of the rays, since suchrays, as common with I all forms of wave length emitted from a oint,decrease as the square of the distance.

since this discovery t has been customary to judge of the amount andintensity of the X-rays emitted from the tube by observing of thefluorescence produced on such salts y means of a rimitive device knownas a fluoroscope. he accuracy of this measurement, however," as in theother case, depends entirely upon the condition of the eyes of theobserver. In. carr ing out my invention, I make use of this iscoverythat X-rays produce fluorescence in certain salts, in connection with a.further well known scientific fact that the ohmic resistance of 'aselenium cell is directly in roportion to the intensit of the light towhich said hat I measure is the quan- -rays.'or Roentgen ra s, andwhilethere are manyot ertferms of radiant "tte d by the tub'efstill it is'suflipractical-purposes to be able 'to accurately, continuously, andsafely the tions from' the tube.- v r r I 'In carrying out my invention,I make use of apparatus such as shown in the accompanying drawing, inWhJOh the X-ray tube is shown at 1; the fluorescent screen of tungstateof calcium or similar material at 2, and

in suitable proximity to the X-ray tube to re-- ceive the emanationsfrom the latter. This fluorescent material may either be placed in alightimpervious container, or box 3, or

may be used as a screento cover an o enin'g in said box, as shown.Contained 1n the light-tight box andfacing the screen 2 1s theenium ce,4, the whole being mounted X'-'rays,-flwhich are the principalemanaupon a suita le support or stand 5, so that it I can be placed .atthe necessary fixed or uniform distance from the X-ray tube.

The selenium cell4, as is well known, has

- fluorescence, to strike said ce its ohmic re-' m the circuits.

for measur".

the roperty of changing its ohmic resistance to t e assage of electriccurrent with variation of ight to which it is exposed. In total darknesssuch a cell may have a resistance of several hundred ohms, yet uponpermitting light, either natural or artificial, reflected or direct, orthe result of phos horescence or sistance. falls almost instantly'to agreat degree, and the variation in its ohmic resistance thus producedbears a direct relation to the intensity of the light. In m arrangementthis selenium cell faces the uorescent material 2, and since the latterbecomes fluorescent under the emanation's of the X-ray tube,

. and such fluorescence is in direct ratio to the intensity of theseemanations, it is at once obvious that the resistance of the seleniumcell varies directly as the intensity of the emanations. from the tube.If, therefore, we measure the change of ohmic resistance of the seleniumcell, we et a direct measurement of the intensity 0 certain emanationsfrom the tube.

Various forms of apparatus may be used ance of the se enium'cell. Forthis purpose I connect said cell in an electric circuit 8, containing asuitable source of power, such as the battery 9, and also connect insaid circuit a suitable instrument for measuring the current in thecircuit, such as a galvanometer 10, having a pointer traveling over agraduated scale. It is obvious that the light-tight box containing theselenium cell may be placed at any desired distance from the remainderof the a paratus. In making these measurements, 1t is of coursenecessary thatthe same conditions exist at each reading, both as tostrength of currentin the circuit 8, and the condition of the seleniumcell 4, which may be affected by external light conditions, or othercauses. In'order to secure accuracy, it is therefore necessa to haveproper adjusting and balancing evices or resistances I therefore placeinthe circuit in series with the selenium cell and galvanometer or meter10, a suitable variable resistance, or rheostat 12, by means of whichmore or less ohmic resistance can be placed in the circuit, so that byproper adjustment of this, a point is reached in which the ohmicresistance of this rheostat, the connecting wirrr, the measuringinstrument 10, and the selenium cell 4 when in darkness, exactlybalances the electro-force on the circuit. Consequently no current willpass the measuring instrument or galvanometer, and the ointer of thelatter will stand at zero. It is further necessary that the current bethe same for each reading. Inasmuch as no known source of current isabsolutely constant, I have provided a battery testing circuit14, inwhich is placed a volt meter 15, and a variable resistance or rheostat16, the

the variations of ohmic resist-- seaaee latter also being in the circuit8. By means of the latter, enough ohmic resistance can be put in thecircuit to et a fixed uniform readmg, say 2 volts on t e volt meter 15at each meet the apparatus. In this battery circuit is placed a suitableswitch 17, b the closing of which a readin or testing 0 the battery canbe quickly 0 tained, andv the variable resistance 16 adjusted to bringthe current from the battery down to a fixed uniform value. The circuit8 is also provided with a suitable switch 18, which preferably will beoperated pneumatically so that it can be instantly opened and closed.

In the use of the a paratus described, the battery is first tested byclosing the switch 1 7, and the rheostat 16 is adjusted until the fixeduniform current value is reached. Then the switch 18 is closed, and therheostat 12 adjusted until the resistance of said rheostat, theconnecting wires, the meter 10, and the selenium cell practicallybalance the electro motive force of the battery 9, or at leastsufficiently to reduce the current in the circuit to such an extent thatit will produce no effect on the meter 10. Consequently the pointer ofsaid meter will stand at zero. The

uorescent screen 2 is then exposed to the emanations of the X-ray tubeat a uniform or fixed distance. The effect of the X-rays is to rendersaid fluorescent material luminous, thus illuminating the interior ofthe light-tight container 3, and reducing the ohmic resistance of theselenium cell 4. This reduces the ohmic resistance in the circuit 8,destroys the balance and permits a current to flow through the meter 10,the quantity of which is indicated by the hand on the graduated scale.Inasmuch as the de ree of fluorescence is directly proportionaI to theintensity of the X-rays, and the decrease in ohmic resistance of theselenium cell is in direct ratio to the intensity of light thrownthereon from the fluorescent screen, it is at once obvious that thereading on the meter 10 indicates accurately the intensity and quantityof the X-rays. This reading is obtained instantly, without calculationsor analyses, without any personal equations whatsoever, and in a mannernot to expose the observer to physical discomforts or injury.

All X-ray tubes emit rays of various degrees of penetration. A tube ofhigh eiiiciency emits rays which are mostly of high penetrating powerand only a few of low iplenetrating power, while in a tube of lowefciency the reverse is true. Since only the rays of hi h penetrationare of value, and since both liinds produce equal luminosity of thefluorescent screen, provision is made to cut 0 or filter out the rays oflow penetration. This I accomplish by lacing in the path of the rays ascreen 20 or aluminum or X-rays. The eflect of this screen 1s to cut Ioff the rays of low (penetration. Preferably .this screen is of wegeshape,;;or concave, or convex, or other shape having differentportions of differentthicknesses, so that a portion of the rays of lowpenetration may pass in order to get some reading'from tubes ofextremely low efficiency. The effect of the;

graduated screen is to give a graded reading, and one of greaterdelicacy at the lower end of the scale.

It is obvious that in place 'ofhaving the fluorescent material in theform of a screen in the li ht-tight box 3, said material may be saidcell. s z

2. Apparatus for. measurin radiant enrescent material arranged to castits light on ergy, comprising material whio becomes flu-- orescent underthe action of the radiant en' er y, a selenium cell protected fromexternal hg t and arranged to be acted on by the fluo: rescence of saldmaterial, and means for measuring the resistance of said selenium.

cell.

3. Apparatus for measurin radiant en- .erg'y, comprisin materialt-whicbecomes lummous under t e action of the' radiantenergy, a selemumcell-in position to be acted on by the fluorescent material, a lightimper- I vious container for saidcell, an e ectric circuit in which saidcell is placed, and a meter in said circuit.

4. Apparatus for measurin radiant energy, comprising material whichbecomes finorescent under the'action of the radiant energy, and meansfor measuring the degree of said fluorescence.

5. Apparatus for measurin ergy, comprising material whic becomesfluorescent under the action of said energy, and light measuringapparatus.

6. Apparatus or measurin radiant energy, comprising material whicbecomes fluorescent under the action of said radiant energy, means formeasuring the degree of said fluorescence, and means protecting thefluorescent material from rays of low penetra tion.

7. Apparatus for measuring radiant energy, comprising fluorescentmaterial, a graduated screen of relatively radiant opaque material infront of the fluorescent material, a selenium cell, and means formeasuring the resistance of said cell.

8. Apparatus for measuri radiant energy, comprising material whicbecomes fluorescent under the action of said radiant energy, a seleniumcell, means for measuring the resistance of said cell, and a relativelyradiant-opaque screen in front of the fluorescent material. v

.9. Apparatus for measurin radiant energy, comprising a selenium ce 1,fluorescent material, a light impervious container for said cell andmaterial, an electric circuit containing said cell, a source of energy,a meter radiant enand a rheostat, a volt meter in circuit with thesource of energy, and a rheostat in said GEORGE o'. JOHNSTON.

Witnesses v F. W. WINTER,

ROBERT C. TOTTEN.

